Re: [RFC v5 PATCH 0/9] xfs: automatic relogging experiment

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Feb 27, 2020 at 08:43:12AM -0500, Brian Foster wrote:
> Hi all,
> 
> Here's a v5 RFC of the automatic item relogging experiment. Firstly,
> note that this is still a POC and experimental code with various quirks.

Heh, funny, I was going to ask you if you might have time next week to
review the latest iteration of the btree bulk loading series so that I
could get closer to merging the rest of online repair and/or refactoring
offline repair.  I'll take a closer look at this after I read through
everything else that came in overnight.

--D

> Some are documented in the code, others might not be (such as abusing
> the AIL lock, etc.). The primary purpose of this series is still to
> express and review a fundamental design. Based on discussion on the last
> version, there is specific focus towards addressing log reservation and
> pre-item locking deadlock vectors. While the code is still quite hacky,
> I believe this design addresses both of those fundamental issues.
> Further details on the design and approach are documented in the
> individual commit logs.
> 
> In addition, the final few patches introduce buffer relogging capability
> and test infrastructure, which currently has no use case other than to
> demonstrate development flexibility and the ability to support arbitrary
> log items in the future, if ever desired. If this approach is taken
> forward, the current use cases are still centered around intent items
> such as the quotaoff use case and extent freeing use case defined by
> online repair of free space trees.
> 
> On somewhat of a tangent, another intent oriented use case idea crossed
> my mind recently related to the long standing writeback stale data
> exposure problem (i.e. if we crash after a delalloc extent is converted
> but before writeback fully completes on the extent). The obvious
> approach of using unwritten extents has been rebuffed due to performance
> concerns over extent conversion. I wonder if we had the ability to log a
> "writeback pending" intent on some reasonable level of granularity (i.e.
> something between a block and extent), whether we could use that to
> allow log recovery to zero (or convert) such extents in the event of a
> crash. This is a whole separate design discussion, however, as it
> involves tracking outstanding writeback, etc. In this context it simply
> serves as a prospective use case for relogging, as such intents would
> otherwise risk similar log subsystem deadlocks as the quotaoff use case.
> 
> Thoughts, reviews, flames appreciated.
> 
> Brian
> 
> rfcv5:
> - More fleshed out design to prevent log reservation deadlock and
>   locking problems.
> - Split out core patches between pre-reservation management, relog item
>   state management and relog mechanism.
> - Added experimental buffer relogging capability.
> rfcv4: https://lore.kernel.org/linux-xfs/20191205175037.52529-1-bfoster@redhat.com/
> - AIL based approach.
> rfcv3: https://lore.kernel.org/linux-xfs/20191125185523.47556-1-bfoster@redhat.com/
> - CIL based approach.
> rfcv2: https://lore.kernel.org/linux-xfs/20191122181927.32870-1-bfoster@redhat.com/
> - Different approach based on workqueue and transaction rolling.
> rfc: https://lore.kernel.org/linux-xfs/20191024172850.7698-1-bfoster@redhat.com/
> 
> Brian Foster (9):
>   xfs: set t_task at wait time instead of alloc time
>   xfs: introduce ->tr_relog transaction
>   xfs: automatic relogging reservation management
>   xfs: automatic relogging item management
>   xfs: automatic log item relog mechanism
>   xfs: automatically relog the quotaoff start intent
>   xfs: buffer relogging support prototype
>   xfs: create an error tag for random relog reservation
>   xfs: relog random buffers based on errortag
> 
>  fs/xfs/libxfs/xfs_errortag.h   |   4 +-
>  fs/xfs/libxfs/xfs_shared.h     |   1 +
>  fs/xfs/libxfs/xfs_trans_resv.c |  24 +++-
>  fs/xfs/libxfs/xfs_trans_resv.h |   1 +
>  fs/xfs/xfs_buf_item.c          |   5 +
>  fs/xfs/xfs_dquot_item.c        |   7 ++
>  fs/xfs/xfs_error.c             |   3 +
>  fs/xfs/xfs_log.c               |   2 +-
>  fs/xfs/xfs_qm_syscalls.c       |  12 +-
>  fs/xfs/xfs_trace.h             |   3 +
>  fs/xfs/xfs_trans.c             |  79 +++++++++++-
>  fs/xfs/xfs_trans.h             |  13 +-
>  fs/xfs/xfs_trans_ail.c         | 216 ++++++++++++++++++++++++++++++++-
>  fs/xfs/xfs_trans_buf.c         |  35 ++++++
>  fs/xfs/xfs_trans_priv.h        |   6 +
>  15 files changed, 399 insertions(+), 12 deletions(-)
> 
> -- 
> 2.21.1
>